The Jerusalem Jews whitewashed grave markers just before Passover to alert pilgrims to their presence. They did this so these strangers would not unknowingly touch one, become unclean, and therefore be ineligible to participate in the feast. [1] It was not so much the whitewashing that made them attractive as it was the monuments themselves that were attractive. Jesus compared these whitewashed monuments to the Pharisees. Both appeared attractive, but both also contaminated people who contacted them. Pharisaic contamination precluded participation in the blessings that Passover anticipated, namely, kingdom blessings. [source][source][source]
Jesus" mention of "lawlessness" is significant ( Matthew 23:28). The Pharisees prided themselves on punctilious observance of the Law (Gr. nomos). Ironically their failure to understand and apply the Law correctly made them lawless (Gr. anomia) in Jesus" view. Anomia is a general word for wickedness in the New Testament. Jesus implied that the Pharisees" whole approach to the Law was really wicked. [source][source][source]